This week we considered the physicality of artworks, approaching them from the point of view of the materials of their construction. It is an odd quirk of museum caption-writers that they almost always include a list of the materials from which the work has been made (a convention that gets tricky and more interpretative when it comes to the conceptual end of the spectrum - what, for example, is the medium of Martin Creed's The Lights Going on and Off(answer: an electrical timer - but that doesn't seem quite accurate as a description), or even of Tracy Emin's My Bed (answer: matress, linen, pillows, objects - again, doesnt seem quite right - why are the pillows given the status of readymades, but not the sheets?). Is Jeff Koons' Three Ball Total Equilibrium Tank made from three basketballs and liquid and perspex, or is it made from rubber, air, liquid and perspex? (intriguingly Koons used different makes of basketball for different versions - should that be part of the caption?).

Why might we be so interested in the medium anyway? Certainly, the knowledge provided by the caption (which we take to be an accurate factual description) colours our interpretation and creates expectation.

Answers include:

The general point that information of this kind helps us understand what we are looking at and what sort of choices the artist has made.

It might not always be possible to work out what the medium is just by looking - e.g. Jeff Koons' 'inflatables' look as if they are ballons - knowing they are constructed from stainless steel makes a different to our interpretation of them (as a kind of trompe l'oeil, rather than readymades) - see for example his famous Rabbit. If you accept Bernard Berenson's tactile theory of art, you might even think that such knowledge affects your muscular reactions to what you are seeing

[Digression:

The art historian and connoisseur Bernard Berenson writing about Giotto in his book Italian Painters of the Renaissance, set out his view about tactile values. For him effective figure painting needed 'the illusion of being able to touch the figure'. He wrote:

'I must have the illusion of varying muscular sensations inside my palm and fingers corresponding to the various projections of this figure, before I shall take it for granted as real, and let it affect me lastingly.'

The critic Roger Fry in his 'Essay on Aesthetics' of 1909 suggested that the emotional effects of painting are felt, that rhythm in visual art 'appeals to all the sensations which accompany muscular activity'. Similarly mass, and space depicted in visual art appeal to kinetic and tactile qualities. These ideas might be extended to thinking about moving and touching the surface of sculptures - when actual touch is impossible, imagined touch is based in part on our understanding of what it is that we might touch - in the case of Koons' 'inflatables', stainless steel, rather than plastic. For more on touching and imagining artworks, see these notes from a previous course which included a Tate Modern touch tour ] - end of digression!

Once we know that we are looking at a work in a certain medium, we are likely to scrutinize that work in a different way, seeing features of the work that we might otherwise overlook. The knowledge shapes our seeing (which is never neutral). Knowing that a Chuck Close or Gerhard Richter painting is a painting in oil rather than a photograph is important to our assessment of it.

More cynically, caption writers often aim at a degree of neutrality. When writing miminal captions, and, generally, specifying media is an uncontroversial factual matter - an easy option, then, when compared with interpretative writing.

There may, too, be historical explanations of where this captioning convention came from (early museums were keen to separate the natural from the products of human ingenuity). But showing the origins doesn't necessarily justify this widespread practice now.

(More ideas welcome)

In Tate Modern we focussed on a range of Arte Povera works, paying close attention to the textures, and juxtapositions of materials. All the works foregrounded medium - none were overtly representational. The choice of medium was something that the viewer was intended to recognize as an artistic decision - the deliberate use of non-conventional raw materials of lumps of concrete and wire, carpet underfelt, a timber beam, latex, a sheet and a glass vessel. These unconventional choices, however, would be obvious to the viewer independently of captioning. By presenting these juxtapositions of materials as artworks, the artists draw attention to features of the materials that would be easy to overlook, make them strange again, and somehow poetic.

One work stood out. Giuseppe Penone's Tree of 12 Metres in which he re-discovered a younger form of a tree within a huge timber beam by carving back through the knots to reveal the branches and trunk as they were in one year of the tree's life. Here he shaped the unpromising medium of a cracking rough-hewn timber beam into a work of art that draws attention to previously unseen aspects of the wood - the inner tree (a process of revelation that has obvious resonances with the way our pasts are carried within us). It is not as if this work could have been expressed in a related medium (unless that happened to be a different type of timber) - the work was only possible in that medium, and it is the medium itself which is in large part the subject matter.